Characterizing erosion processes on a convex slope based on 3D reconstruction method

THE National Resource Inventories (NRI) carried out by the Soil Conservation Service in 1977 provide statistical data on sheet and rill erosion for all nonfederal land in the United States (excluding Alaska) and data on wind erosion in 10 Great Plain States. The data were ob-tained by visiting plots and randomly selected points to gather information for use in the Universal Soil Loss Equation and Wind Erosion Equation models. The NRI show that soil lost by sheet and rill erosion annually amounts to 3.5 billion metric tons (t). About one-half of this loss is from the 167-million-ha cropland base; the most sloping 26 percent contributes 79 percent of the cropland erosion. In 19 states and the Caribbean Area, estimated average annual cropland sheet and rill erosion exceeds 11.2 t/ha. In four states and the Caribbean Area the rate exceeds 22.4 t/ha. The 219 million ha of nonfederal pastureland and rangeland account for 1.4 billion metric tons of soil lost from sheet and rill erosion. The loss on nonfederal forest land is about 395 million t; the average annual rate is 2.6 t/ha, but the rate is six times higher on grazed than on nongrazed forest land. Soil lost by wind erosion, estimated for 209 million ha of land in 10 Great Plains States, annually amounts to 1.4 billion t. About 61 percent of this loss is from cropland and 38 percent is from rangeland. The estimated average annual rate of wind erosion on cropland exceeds 11.2 t/ha in three of the 10 states.

Runoff scouring experimental study of rill erosion of spoil tips

Field experiments were conducted on various runoff plots within the Zhangjiachong watershed, located in the Three Gorges Reservoir Area, to study rainfall‐runoff processes and sediment yields under different slope land use patterns. A hillslope run‐on model considering vegetation interception was used to simulate runoff generation and concentration. A slope erosion model including raindrop erosion, sheet erosion, and rill erosion was developed to estimate sediment yield and concentration. The parameters of both models were calibrated and validated using observed runoff and sediment yield data. The results show that the runoff simulations in the validation have relative errors below 20% for all runoff plots, except those planted with crops. The saturated hydraulic conductivity of soil was found to be ca. 10% higher on slope lands with hedgerows than those without. In addition, it was about 15% higher on slope lands with crops than on slope lands with tea plants and citrus. The sediment yield was shown to be influenced by both precipitation characteristics and antecedent sediment conditions. There was a significant relationship between sheet erosion, rill erosion, and vegetation coverage, while the amount of raindrop erosion was similar across different runoff plots. The proportions of sheet and rill erosion on slope land with crops were minimal whether hedgerows existed or not. The proportions of the three main forms of soil erosion on slope lands with tea plants and citrus were more or less the same under concentrated rainfall. For more evenly distributed rainfall, raindrop erosion was the primary form on slope lands with hedgerows‐tea plants and citrus. The proportions of sheet and rill erosion significantly increased on slope lands without hedgerows.

Impacts of rainfall and inflow on rill formation and erosion processes on steep hillslopes

Seasonal freeze-thaw processes have led to severe soil erosion in the middle and high latitudes. The area affected by freeze-thaw erosion in China exceeds 13% of the national territory. So understanding the effect of freeze-thaw on erosion process is of great significance for soil and water conservation as well as for ecological engineering. In this study, we designed simulated rainfall experiments to investigate soil erosion processes under two soil conditions, unfrozen slope (UFS) and frozen slope (FS), and three rainfall intensities of 0.6, 0.9 and 1.2 mm/min. The results showed that the initial runoff time of FS occurred much earlier than that of the UFS. Under the same rainfall intensity, the runoff of FS is 1.17–1.26 times that of UFS; and the sediment yield of FS is 6.48–10.49 times that of UFS. With increasing rainfall time, rills were produced on the slope. After the appearance of the rills, the sediment yield on the FS accounts for 74%–86% of the total sediment yield. Rill erosion was the main reason for the increase in soil erosion rate on FS, and the reduction in water percolation resulting from frozen layers was one of the important factors leading to the advancement of rills on slope. A linear relationship existed between the cumulative runoff and the sediment yield of UFS and FS (R2>0.97, P<0.01). The average mean weight diameter (MWD) on the slope erosion particles was as follows: UFS0.9 (73.84 µm)>FS0.6 (72.30 µm)>UFS1.2 (72.23 µm)>substrate (71.23 µm)>FS1.2 (71.06 µm)>FS0.9 (70.72 µm). During the early stage of the rainfall, the MWD of the FS was relatively large. However, during the middle to late rainfall, the particle composition gradually approached that of the soil substrate. Under different rainfall intensities, the mean soil erodibility (MK) of the FS was 7.22 times that of the UFS. The ratio of the mean regression coefficient C2 (MC2) between FS and UFS was roughly correspondent with MK. Therefore, the parameter C2 can be used to evaluate soil erodibility after the appearance of the rills. This article explored the influence mechanism of freeze-thaw effects on loess soil erosion and provided a theoretical basis for further studies on soil erosion in the loess hilly regions.

Rill erosion has serious impacts on arable soils in Chinese Loess Plateau. It is imperative to clarify the key processes of rill erosion to implement effective soil conservation practices. In this study, rill erosion on a 26.8% slope was evaluated using a Terrestrial Laser Scanner (TLS) and CA-Rill model under simulated rainfall. The results showed that (1) the TLS method and the CA-Rill model could effectively describe slope erosion processes, including the splash and sheet erosion, drop pits, intermittent rills, and rill network development stages; (2) compared with the traditional method, the average error of the CA-Rill model for estimating runoff and soil loss was 6.8% and 26.8%, respectively, the CA-Rill model could accurately simulate runoff processes, but poorly simulated sediment processes, especially in splash and sheet erosion stages; and (3) rill erosion evolved into the dominant erosion pattern after 30 min of rainfall, then rill morphological parameters – rill plane density, average rill depth, maximum rill length, and average rill width enlarged, and increased by more than 3.7, 1.8, 2.9, and 4.0 times by the end of rainfall. The methods used to estimate sediment yields and the variation of morphological parameters could provide reference for soil erosion monitoring and modelling.

Effects of antecedent soil moisture on rill erodibility and critical shear stress

The Daily Erosion Project (DEP) began about 20 years ago as a daily sheet and rill erosion estimator for the state of Iowa, USA by using weather data from US National Weather Service NEXRAD radar and automated weather stations as well as USDA National Resource Inventory (NRI) survey data accessed at the township level and Soil Survey Geographic (SSURGO) soil properties data. In 2014 we revised our framework to use remote sensing methods to gather all inputs at the field level. These efforts have led to the development of the Agricultural Conservation Planning Framework, https://acpf4watersheds.org, a spatial database and geoprocessing toolbox that allows one to generate candidate locations for site specific conservation practices. The addition of automated Digital Elevation Model (DEM) enforcement techniques allowed development of a new sampling strategy to estimate sheet and rill erosion on hundreds of thousands of agricultural flowpaths across the Midwestern United States; https://dailyerosion.org. This is done by using the US 12-digit Hydrologic Unit Code (HUC) basin numbering system in conjunction with an even finer system of delineation to define approximately 100 ha DEP subcatchments. One agricultural flowpath is randomly selected and modeled in each subcatchment and the mean of the HUC12 is the estimated erosion. However, DEP estimates still do not include ephemeral gully or channel erosion processes and only occur on agricultural lands, thus DEP still lacks simulation of many erosion processes. Our objective in this project is to develop methods to add sheet and rill erosion estimates on forested lands and channel erosion estimates on agricultural lands to DEP. Considered alone, the addition of sheet and rill erosion forest estimates is somewhat straightforward as many inputs remain the same. However, the basic land management unit used by the DEP processing pipeline is the agricultural field, so expanding to include forested areas required developing new algorithms. To achieve this, we started with the LANDFIRE canopy cover database and street map datasets from the US Forest Service (USFS) and Open Streetmaps, respectively. LANDFIRE specifies the amount of canopy cover on forested lands throughout the United States, enabling selection of appropriate forest management for each 30m pixel from USFS files developed for each 10% canopy cover increment. Additionally, forest erosion is highly sensitive to channelization due to road networks, so hillslope erosion estimates are truncated at roads to keep the modeled areas constrained to places where sheet and rill erosion processes dominate. Last, modeled hillslopes are considered either agricultural or forested, but not both (agricultural hillslopes truncate at forest and forested hillslopes truncate at agricultural fields) to maintain estimates for each part separately. Adding channel erosion estimates to DEP is also necessary to adequately estimate overall erosion but requires alteration of current DEP practices to use the watershed version of WEPP (rather than hillslope). In furtherance of this goal, new algorithms are being developed to define and parameterize basins inside DEP subcatchments, develop flow networks for the basins, parameterize flow network channel geometry from QL2 LiDAR and to map land cover and management in channels as most datasets do not resolve vegetated waterways. Basin and flow network delineation can be accomplished by setting critical grid order thresholds similar to the current DEP process for hillslope termination. Model results for forest erosion are consistent with expectations but extensive field validation presents difficulties. Model results can be combined with the grid order threshold process to identify areas of potential channel erosion and the relative likelihood that erosion will occur in a given channel. The addition of these estimates to DEP helps further soil degradation process modeling and will enable future analyses into erosion forms and dynamics across different temporal and geological scales.

Predicting soil erosion and deposition on sloping farmland with different shapes in northeast China by using 137Cs

Extensive areas in the Valley of Mexico are covered by highly erodible volcanic soils.The traditional methods of managing these soils with deep ripping and cultivation haveincreased soil erosion. Soil erosion is greatly influenced by the interactions between rainfall andsoil characteristics and therefore, the study of interrill and rill erosion can be very helpful indeveloping new approaches for controlling soil erosion in order to maintain sustainableproduction and environmental integrity. The objective of this study was to evaluate the effect ofgypsum on interrill erosion and the effect of polyacrylamide on rill erosion in a highly erodiblevolcanic soil from the Valley of Mexico using rainfall and runoff simulation techniques. In theinterrill area, surface application of gypsum increased infiltration and reduced runoff, soil erosionand surface sealing. Rill erosion caused by concentrated flow was practically reduced to zerowhen polyacrylamide (PAM) was applied in simulated rills. The use of soil amendments tocontrol soil erosion appears to be a viable management method for reclaiming highly erodiblevolcanic soils at a lower cost.

The soil erosion of the spoil tips seriously threatens the safety of people's lives and property and the surrounding ecological environment. Rill erosion is an important cause of water and soil loss in spoil tips. This study was conducted to investigate the process of rill erosion on the slopes of spoil tips, changes in the morphological characteristics of rills and the mechanisms of rill erosion. A Field runoff plot (5 m long, 1 m wide and 0.5 m deep) with three inflow rates (1.6, 2 and 2.4 mm min−1) and three typical slopes (28°, 32° and 36°) was used for runoff simulation experiments. The results showed that, compared with the slope and scouring times, inflow rate was the most important factor affecting rill erosion of the spoil tips. The development of rill mainly goes through three stages: the rill formation stage, the rill development stage and the rill adjustment stage. The overall predominance of parallel-shaped rills at all experiments suggested that the formation of rills was dominated by concentrated runoff. The average rill depth was the best indicator of rill morphology for evaluating rill erosion. The flow regimes under the experimental conditions were supercritical-laminar flow and supercritical-transition flow. The Reynolds number was the best hydraulic parameter for predicting rill erosion. The stream power was the best hydrodynamic parameter to describe rill erosion mechanism. These results contributed to further revealing the rill erosion mechanism on the slope of the spoil tips and provided a scientific basis for its soil erosion control.

The soil erosion of the spoil tips seriously threatens the safety of people's lives and property and the surrounding ecological environment. Rill erosion is an important cause of water and soil loss in spoil tips. This study was conducted to investigate the process of rill erosion on the slopes of spoil tips, changes in the morphological characteristics of rills and the mechanisms of rill erosion. A Field runoff plot (5 m long, 1 m wide and 0.5 m deep) with three inflow rates (1.6, 2 and 2.4 mm min−1) and three typical slopes (28°, 32° and 36°) was used for runoff simulation experiments. The results showed that, compared with the slope and scouring times, inflow rate was the most important factor affecting rill erosion of the spoil tips. The development of rill mainly goes through three stages: the rill formation stage, the rill development stage and the rill adjustment stage. The overall predominance of parallel-shaped rills at all experiments suggested that the formation of rills was dominated by concentrated runoff. The average rill depth was the best indicator of rill morphology for evaluating rill erosion. The flow regimes under the experimental conditions were supercritical-laminar flow and supercritical-transition flow. The Reynolds number was the best hydraulic parameter for predicting rill erosion. The stream power was the best hydrodynamic parameter to describe rill erosion mechanism. These results contributed to further revealing the rill erosion mechanism on the slope of the spoil tips and provided a scientific basis for its soil erosion control.

The indoor fine simulated rainfall experiment was conducted to reveal the variation regularity of the rill boundary and the response of the erosion and sediment yield under the fixed slope gradient (20°) and three rainfall intensities (66, 94, 127 mm/h). Results showed that the response of the slope sedimentation process to the rill boundary changes is obvious. Rill traceability is the most active way of determining sediment yield during rill formation and development; it is the main source of sediment in the early stages of rill development. Rill wall collapse and bed cut are the more active ways of sediment yield during rill network development and formation; they directly lead to increase in soil erosion rate in the middle stages of rill development. Rill bed cut is the more active way of sediment yield during rill network adjustment. The change in the relationship between runoff shear and sediment concentration directly leads to the continuous fluctuation of soil erosion rate in the later stage of erosion. The sand production is affected by the development of the rill. There is a correspondence between the rill network at different developmental stages and the sediment yield. The sediment concentration of runoff is relatively low before the formation of the rill, and then fluctuates continuously in the rill development stage and tends to be relatively stable in the stable development stage; while the sediment concentration of runoff is proportional to the rain intensity. This study can provide basic data for the establishment of rill erosion dynamic model.

Concentrated runoff increases erosion and moves fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on croplands in the U.S. may contribute more of the sediment delivered to the edge of the field then from sheet and rill erosion. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but science and technology are needed to account for the separate benefits and effects of practices on each of the various sediment sources.Watershed modeling technology has been widely developed to aid in evaluating conservation practices implemented as part of a management plan, but typically lacks the capability to identify how a source, such as sheet and rill erosion, ephemeral gully erosion, or channel erosion, is specifically controlled by a practice or integrated practices. The U.S. Department of Agriculture&amp;#8217;s Annualized Agricultural Non-Point Source pollutant loading model, AnnAGNPS, has been developed to determine the effects of conservation management plans on erosion and provide sediment tracking from all sources within the watershed, including sheet and rill, ephemeral gully, and channel erosion.&amp;#160;This study describes the ephemeral gully erosion capabilities within the AnnAGNPS model and discusses research needs to further improve these components for integrated conservation management planning. &amp;#160;Conservation management planning by agencies within the U.S. and by international organizations requires a systematic approach when determining the extent of ephemeral gully erosion impacts on a field, watershed, or national basis, and/or to predict recurring or new locations of ephemeral gullies prior to their development.&amp;#160; This technology provides the capability to separate the impact of ephemeral gullies on erosion from other sources and then evaluate the impact of targeted practices to control erosion at the source and subsequent downstream resources.

Introduction to the special issue ‘Tracer Applications in Sediment Research’